<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Timer.4 - Using a member function as a handler</title>
    <link rel="stylesheet" href="../../boostbook.css" type="text/css">
    <meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
    <link rel="home" href="../../index.html" title="Asio">
    <link rel="up" href="../tutorial.html" title="Tutorial">
    <link rel="prev" href="tuttimer3/src.html" title="Source listing for Timer.3">
    <link rel="next" href="tuttimer4/src.html" title="Source listing for Timer.4">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%">
    <tr>
        <td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td>
    </tr>
</table>
<hr>
<div class="spirit-nav">
    <a accesskey="p" href="tuttimer3/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u"
                                                                                           href="../tutorial.html"><img
        src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png"
                                                                                      alt="Home"></a><a accesskey="n"
                                                                                                        href="tuttimer4/src.html"><img
        src="../../next.png" alt="Next"></a>
</div>
<div class="section">
    <div class="titlepage">
        <div>
            <div><h3 class="title">
                <a name="asio.tutorial.tuttimer4"></a><a class="link" href="tuttimer4.html"
                                                         title="Timer.4 - Using a member function as a handler">Timer.4
                - Using a member function
                as a handler</a>
            </h3></div>
        </div>
    </div>
    <p>
        In this tutorial we will see how to use a class member function as a callback
        handler. The program should execute identically to the tutorial program from
        tutorial Timer.3.
    </p>
    <pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span
            class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span
                class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span
                class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span
                class="identifier">boost</span><span class="special">/</span><span class="identifier">bind</span><span
                class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span
                class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
    <p>
        Instead of defining a free function <code class="computeroutput"><span class="identifier">print</span></code>
        as the callback handler, as we did in the earlier tutorial programs, we now
        define a class called <code class="computeroutput"><span class="identifier">printer</span></code>.
    </p>
    <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">printer</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
    <p>
        The constructor of this class will take a reference to the io_context object
        and use it when initialising the <code class="computeroutput"><span class="identifier">timer_</span></code>
        member. The counter used to shut down the program is now also a member of
        the class.
    </p>
    <pre class="programlisting">  <span class="identifier">printer</span><span class="special">(</span><span
            class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span
            class="special">&amp;</span> <span class="identifier">io</span><span class="special">)</span>
    <span class="special">:</span> <span class="identifier">timer_</span><span class="special">(</span><span
                class="identifier">io</span><span class="special">,</span> <span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span
                class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span
                class="special">)),</span>
      <span class="identifier">count_</span><span class="special">(</span><span class="number">0</span><span
                class="special">)</span>
  <span class="special">{</span>
</pre>
    <p>
        The <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a> function
        works just as well with class member functions as with free functions. Since
        all non-static class member functions have an implicit <code class="computeroutput"><span
            class="keyword">this</span></code>
        parameter, we need to bind <code class="computeroutput"><span class="keyword">this</span></code>
        to the function. As in tutorial Timer.3, <a class="link" href="boost_bind.html"
                                                    title="boost::bind">boost::bind</a>
        converts our callback handler (now a member function) into a function object
        that can be invoked as though it has the signature <code class="computeroutput"><span
            class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span
            class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span
            class="special">&amp;)</span></code>.
    </p>
    <p>
        You will note that the asio::placeholders::error placeholder is not specified
        here, as the <code class="computeroutput"><span class="identifier">print</span></code> member
        function does not accept an error object as a parameter.
    </p>
    <pre class="programlisting">    <span class="identifier">timer_</span><span class="special">.</span><span
            class="identifier">async_wait</span><span class="special">(</span><span class="identifier">boost</span><span
            class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span
            class="identifier">printer</span><span class="special">::</span><span class="identifier">print</span><span
            class="special">,</span> <span class="keyword">this</span><span class="special">));</span>
  <span class="special">}</span>
</pre>
    <p>
        In the class destructor we will print out the final value of the counter.
    </p>
    <pre class="programlisting">  <span class="special">~</span><span class="identifier">printer</span><span
            class="special">()</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Final count is "</span> <span class="special">&lt;&lt;</span> <span
                class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span
                class="special">;</span>
  <span class="special">}</span>
</pre>
    <p>
        The <code class="computeroutput"><span class="identifier">print</span></code> member function
        is very similar to the <code class="computeroutput"><span class="identifier">print</span></code>
        function from tutorial Timer.3, except that it now operates on the class
        data members instead of having the timer and counter passed in as parameters.
    </p>
    <pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">print</span><span
            class="special">()</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span
                class="special">&lt;</span> <span class="number">5</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
      <span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>

      <span class="identifier">timer_</span><span class="special">.</span><span
                class="identifier">expires_at</span><span class="special">(</span><span class="identifier">timer_</span><span
                class="special">.</span><span class="identifier">expiry</span><span class="special">()</span> <span
                class="special">+</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">chrono</span><span class="special">::</span><span
                class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span
                class="special">));</span>
      <span class="identifier">timer_</span><span class="special">.</span><span
                class="identifier">async_wait</span><span class="special">(</span><span
                class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span
                class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span
                class="identifier">print</span><span class="special">,</span> <span class="keyword">this</span><span
                class="special">));</span>
    <span class="special">}</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span
                class="identifier">timer_</span><span class="special">;</span>
  <span class="keyword">int</span> <span class="identifier">count_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
    <p>
        The <code class="computeroutput"><span class="identifier">main</span></code> function is much
        simpler than before, as it now declares a local <code class="computeroutput"><span
            class="identifier">printer</span></code>
        object before running the io_context as normal.
    </p>
    <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span
            class="special">()</span>
<span class="special">{</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span
                class="identifier">io</span><span class="special">;</span>
  <span class="identifier">printer</span> <span class="identifier">p</span><span class="special">(</span><span
                class="identifier">io</span><span class="special">);</span>
  <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span
                class="special">();</span>

  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
    <p>
        See the <a class="link" href="tuttimer4/src.html" title="Source listing for Timer.4">full source listing</a>
    </p>
    <p>
        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
    </p>
    <p>
        Previous: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 -
        Binding arguments
        to a handler</a>
    </p>
    <p>
        Next: <a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5
        - Synchronising handlers
        in multithreaded programs</a>
    </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%">
    <tr>
        <td align="left"></td>
        <td align="right">
            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
            </p>
            </div>
        </td>
    </tr>
</table>
<hr>
<div class="spirit-nav">
    <a accesskey="p" href="tuttimer3/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u"
                                                                                           href="../tutorial.html"><img
        src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png"
                                                                                      alt="Home"></a><a accesskey="n"
                                                                                                        href="tuttimer4/src.html"><img
        src="../../next.png" alt="Next"></a>
</div>
</body>
</html>
